Sp138-35 Energy Absorption FRP Reinforced-beams

7/24/2019 Sp138-35 Energy Absorption FRP Reinforced-beams

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Synopsis,

SP 138-35

Flexural Behavior and Energy

Absorption o Carbon FRP

Reinforced Concrete Beams

by T. Kakizawa S. Ohno

and T. Yonezawa

Research and development ofFRP bars and cables for reinforcements of

concrete structure has recently been carried out. The basic behavior of the concrete

members reinforced with these FRP bars has became well understood. However,

there are still

debatable

points in terms of the

design concept

such as the

recommended failure mode or required toughness and ductility. The authors

carried out loading tests of the 6 concrete beams reinforced with carbon FRP bars

and cables in order to discuss the both serviceability and ultimate limit states. The

specimens includes

the RC, PC, PPC

members.

The main factors are bond

properties of the FRP reinforcements and prestress force. The experimental results

show that cracking and deformation behavior vary with the prestress force and

bond property of FRP bars, and that the reasonable serviceability condition will be

achieved by controlling these factors. Also, the failure mode were affected by

these factors and the reinforcing systems, despite these specimens have almost

same reinforcement ratio. In relation

to

the failure mode, the energy absorption,

which is defined as the area enclosed by load-deflection curve, was measured to

discuss the toughness and ductility for the ultimate limit state. The authors

recommend that the design should take into account the toughness based on the

energy absorbed before the maximum load.

Keywords: Absorption; beams supports); cable; carbon; cracking

fracturing); deformation; ductility; failure; fiber reinforced plastics; flexural

strength;

prestressed

concrete; reinforced

concrete

585


2/14

586 Kakizawa Ohno nd Yonezawa

Tadahiro Kakizawa, is a research engineer in Takenaka Research Laboratory. He

received an MS in civil engineering from the University

of

Tokyo. He is currently

working in the area

of

development

of

new structural materials.

Sadatoshi Ohno, is a senior research engineer in the advanced materials group

in

Takenaka Research Laboratory. He received his Ph. D. from the University

of

Sur

rey, U.K. His research interests include fracture mechanism, alkali aggregate reac

tion, fiber reinforced composites, and new structural materials.

Toshio Yonezawa,

is

a chief research engineer in the concrete materials group

in

Takenaka Research Laboratory. He received his Ph.D. from the University of

Manchester Institute University, U.K He has been extensively involved with re

search on corrosion problems

of

steel

in

concrete, high strength concrete, fiber

reinforced concrete and new materials in construction field.

INTORODUCTION

Substantial effort have recently been made to develop fiber reinforced

plastics FRP) bars and cables for reinforcement

of

concrete structures. There

is

great interest

in

the high-strength, rust-free, and non-magnetic properties

of

such

new materials.

With

regard to the design

of

structural members using RP

reinforcement, it has been reported that flexural behavior can be predicted based

on conventional flexural theory for reinforced concrete. However, the members

reinforced with

RP

bars or cables exhibit brittle failure prope11ies since FRP

materials have no plastic region, while conventional reinforced concrete and

prestressed concrete show a ductile failure behavior because of the yield of the

steel reinforcement. From this viewpoint, an appropriate design method for

ultimate limit states

of

concrete members reinforced with FRP still remains

to

be

investigated.

In

related discussion, it has also been reported that the compression

failure mode is preferable for such RP reinforced concrete members, because the

failure

of

the member proceeds more gradually

at

the ultimate state compared with

failures

of

those governed by brittle FRP breakage I).

In

contrast, another opinion

is that alternative design methods which can allow for brittle failureof members

due to FRP breakage should be considered for reasonsof economy and rationality

2).


3/14

FRP Reinforcement 587

On the other hand, various studies have worked to improve the ductility

of

FRP reinforced concrete by controlling bond properties

of

the FRP

reinforcement

or

placing the

FRP

reinforcement in multiple stages 3). Also,

attempts on improving brittle behavior by constra ining the compression zone

of

the reinforced members have been reported 4), 5).

However, the important concern is to secure the required ductility both for the

members and the structures being designed, and in order to do this more thorough

discussion

of

the appropriate design needs

to

be undertaken.

EXPERIMENTS

In this experiment, small beam specimens

of

rectangular section were

adopted as shown in Figure I and reinforced concrete RC), prestressed concrete

PC), and partially prestressed concrete PPC) systems using FRP reinforcement

were tested. Cable strand of carbon fiber reinforced plastics CFRP) were used as

prestressing tendons, and two kind

of

tensile

reinforcement- CFRP

cable strands

and CFRP deformed bars- were used. Tables I and 2 give the physical propetties

of the reinforcing materials and the strength

of

the concrete used, respectively.

Loading tests

were

carried

out on

16

types

of

specimens

with different test

parameters - prestress force, reinforcement type, bonded

or

unbonded tensile

reinforcement, and prestressing cable

as

shown in Table 3. The sectional areas

of

FRP reinforcement given in this table are the nominal cross sectional area

including the resin. Specimen No. I is an ordinary reinforced concrete beam

incorporating deformed steel bars, specimen No. 2

is

an RC beam using FRP

reinforcement

specimens No. 3

through

6

are PC

beams

without tensile

reinforcement, and all

other

specimens are

PPC beams. Specimen

No.

16

although made of the same material as specimen No. 13, was made with a 5 mm

thick permanent form reinforced with polypropylene fiber net to improve in

service propet1ies. Unidirectional loading was applied to all beams, which have a

span

of

170

em

and a moment span

of

30 em. n the tests, load, deflection, strain in

the concrete and reinforcement, and crack width were measured.

RESULTS OF EXPERIMENT ND DISCUSSION

Cracking and Deformation Properties

Table 4 shows the

results of the

loading

tests.

Reasonably

close

agreement was obtained between the measured and calculated values of cracking

load. Figure 2 illustrates the cracking patterns of the loaded beams. Provided that

the service load is about

one

third

of

the maximum load, no appreciable cracking

was recognized in the

PC

and

PPC

specimens and the cracking was very fine even


4/14

588 Kakizawa, Ohno, and Y onezawa

when it did occur. The cracks in specimen No.2 are very small, less than 0 1 mm,

at the service load and it is thought that no serious problems would arise in an

actual application as far as cracking is concemed. However, since the specimens

used here were small, it must be taken into consideration that cracks tend to be

smaller than in actual concrete members.

The distribution

of

the cracks along the beams differed according to the

specimen. When the specimen has no tensile reinforcement in the PC member

specimens No. 3 through 6) or has a partially unbonded tensile reinforcement in

the PPC members, fewer cracks were observed. In the case

of

the unbonded PC

specimen CPC58U), cracks were particularly concentrated in the moment span.

When the working load becomes high over

13

kN) in this specimen, the

deformation is concentrated only at the center, as demonstrated by the deflection

distributions shown in Figure 3. This may cause local secondary stress at the

deformed area and/or frictional damage contacting with the sheath. These effects

are not desirable because FRP reinforcement may break earlier than expected.

On the other hand, specimens with CFRP deformed bars used as tensile

reinforcement show good crack distributions, and have closer crack spacing than

conventional RC members. However, longitudinal splitting cracks were found

along the reinforcement at the ultimate state. These cracks were related to the

dimensions of the specimen, the concrete cover, and the bond properties of the

reinforcement. The bond properties of FRP reinforcement are greatly affected by

their configuration

of

deformation, and this remains an area for further study.

In

specimen No.

16

CPRC38UB-NET), where the permanent form

reinforced with

polypropylene fiber

net was

used, the

cracks were

finely

distributed at a spacing ranging from a few millimeters up to one centimeter,

although these cracks are not illustrated in this paper. This behavior is

advantageous when cracking in the application must be limited, and when the

design calls for a wider range of service conditions.

Ultimate Load and Failure Mode

Figures 4 a)- f) show load-deflection curves for the specimens. The

results for specimens reinforced with FRP differed depending on the type of

reinforcement and the differences in bonding. The obtained curves are not

basically different from those

of earlier reports. When the results for CPRC24BB

YY are compared with those for CPRC24-

YR,

CPRC38BB-YY with


5/14

RP Reinforcement 589

CPRC38BB-YR, and

CPRC38UB-

YY with

CPRC38UB

the ultimate load is

found to be

15

to 20 higher for specimens using CFPR deformed bars rather than

CFRP

cables

as

the

tensile

reinforcement in spite of

the

almost identical

reinforcement system and same reinforcement ratio. The reason for this difference

is thought to be due to be the bonding characteristics of the reinforcement. When

two kinds

of

reinforcement with different bond properties are used simultaneously

in the specimen, the actual working stress may be different from the prediction,

which is based on the assumption that plane sections before bending remain plane

after bending.

The calculated and experimental

values

of ultimate strength

were

compared in

Table

4.

The

calculated ultimate strength was derived from the

requirement

of

strain compatibility and equilibrium

of force

by repeating

calculation for the divided elements

of

the beam section. In this calculation, the

relationship between concrete stress and strain was assumed to be expressed by

Umemura's equation(6), as follows.

0

=

O cu

{ 6.75 ( e (-0.819 e ( -1.218

where =_ _

Ecu

cr

concrete stress,


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590 Kakizawa Ohno

nd

Y onezawa

and the arrangement of prestressing cables and reinforcement.

The failure modes predicted by calculation and the results observed from

the experiments are given in Table 4. Although they agree relatively well, there is

a difference between predicted and experimental results for PPC members with

unbonded prestressing cables. Although the reason for this is not clear, some local

stress may have been induced because the measured strain in the prestressing

cables was less than the nominal failure strain at the time of fracture.

According to earlier work, the compression failure mode is slightly

ductile compared with the reinforcement failure mode in FRP reinforced concrete.

However, in this experiment, no great difference could be seen even in the case of

a compression failure. This is because the FRP reinforcement failed during

compression failure when the reinforcement ratio was close to the balanced failure

state CPC69B and CPC58B) See Figure 4). Considering these points, additional

reinforcement over the amount required to obtain compression failure needs be

incorporated in

order

to ensure a compression failure without

FRP

failure.

However, this still presents problems in terms

of

economical design. t is also

difficult to specify the failure mode in the design, since this would mean taking

account of changes in the material properties over the whole lifetime and

of

the

scatter in fracture strength of the FRP reinforcement.

n

the case

of

PPC members

with

FRP

cables as tensile reinforcement, on the other hand, the beam specimens

deformed by relatively large amounts even after failure of the prestressing cable.

This shows the possibility

of

designing FRP reinforced concrete members with

such deformation behavior by controlling the amount of reinforcement and the

bonding properties.

Energy Absorption

n

terms of design, the ductility required of a structural member should

vary depending on its type, importance, service conditions and so on.

n

the design

for conventional reinforced concrete, the brittle failure of members has been

avoided considering safety. Therefore, normally designed reinforced concrete for

flexural members are ductile, because their energy absorption is due to the capacity

of the steel to deform; that is, they have adequate ductility and great energy

absorption.

n the case of FRP reinforced concrete members, avoidance of the brittle failure

may not be easy and economical. Also, the ductility factor which is usually

expressed as the ratio of the ultimate deformation to the deformation at first yield is

not considered to be proper to assess the characteristics of FRP reinforced


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RP Reinforcement

59

concrete. The ductility factor is a index to express the deformation capacity and

consequently shows the energy absorption. Therefore evaluation of energy

absorption would be very important in the design of FRP reinforced concrete,

although a discussion

of

the proper safety factors has to be continued.

Table 4 and Figure 5 show the absorbed energy, which is defined to be

the area enclosed by the load-deflection curve for each specimen. The energy

absorbed before and after the maximum load is shown separately. The values of

the total energy absorption are unlikely to be affected by the failure mode within

the range of this experiment s conditions. PPC members tended to indicate

greater energy

absorption

than PC despite having the same amount of

reinforcement. This may imply an advantage for PPC. When the absorbed energy

after the maximum load is compared, the PC members show no energy absorption

after failure

of

the

FRP

cable at the ultimate load because only prestressing cables

were placed in the PC members in this experiment.

On the contrary, the unbonded PC specimen CPC58U, which failed in

compressive mode, showed the same energy absorption before the maximum load

as after the maximum load. In PPC members, some energy is absorbed even after

the prestressing cables have failed, since FRP reinforcement could still sustains a

load; it thus allows for further deformation. Also in the case

of

PPC members

using FRP reinforcement and prestressing cable of different bond prope11ies, the

amount of absorbed energy after the maximum load tended to be higher when

CFRP cables were used

as

tensile reinforcement, while the energy absorbed before

the maximum load was higher when CFRP deformed rods were adopted

as

tensile

reinforcement. Thus it is possible to obtain various energy-absorption prope11ies

by controlling the reinforcing system. However, evaluating the energy absorption

after the maximum load is technically difficult (The theoretical calculation is

thought to be possible, but there are some problems in accuracy. ) If the concrete

members are such that energy absorption is not expected, i.e. only vertical loads

act and the member doesn t need resist the earthquake load, it may be meaningless

to evaluate the energy absorption over such a range in the design.

The energy absorption

of

FRP reinforced concrete beams and slabs

under the influence of vertical forces should be evaluated by the total energy

absorption up to the maximum load. In the case of the members where an

evaluation of repeated energy loads, such as earthquake loads, is required, fUI her

detailed studies will be necessary.


8/14

592 Kakizawa Ohno

nd

Y onezawa

CONCLUSIONS

In this study, the cracking behavior and failure properties and the energy

absorption of FRP reinforced concrete have been investigated experimentally,

with the prestress force and bonding properties of the FRP reinforcement taken as

the experimental factors. The failure mode and deformation behavior are found to

change according to the reinforcing system. The absorbed energy s affected by the

reinforcing system, but little by the failure mode, within the range of these

experiments. PPC members absorb more energy than PC in spite

of

the same

amount of reinforcement. Based on these results, design criteria were discussed n

connection with energy absorption and failure mode. t is recommended that the

design should take into account the ductility evaluated for the energy absorbed

before maximum load.

REFERENCES

I) H.Mutsuyoshi, A.Machida, and K.Uehara, Mechanical properties and

design method

of

concrete members reinforced by Carbon Fiber Reinforced

Plastics, Proceedings of the Japan Concrete Institute, 12-1, pp. 1117-1122,

1990.

(2) H.Nakai, K.Mukae, H.Asai, and S.Kumagaya, Analytical study on bending

ultimate state of prestressed concrete beams with FRP rods, Proceedings of the

Japan Concrete Institute, 13-2, pp. 749-754, 1991.

(3) Y.Yamamoto, H.Maruyama, K.Shimizu, and H.Nakamura, Fractural

properties of concrete members with a multi-stage arrangement of CFRP, Japan

Society

of

Ci vii Engineering, Proceedings of the 46th Annual Conference, pp.

238-239, 1991.

(4) M.Odera, T.Maruyama, and Y.Ito, Improvement n compression failure

mode of

RC beams using CFRP rods, Japan Society

of

Civil Engineering,

Proceedings of the 46th Annual Conference, pp. 242-243, 1991.

(5) H.Taniguchi, H.Mutsuyoshi, A.Machida, and T.Kita, A proposal of

Improvement n failure mode

of

PC flexural members using FRP, Japan Society

of Civil Engineering, Proceedings of the 46th Annual Conference, pp. 244-245,

1991.

(6) Umemura, Uitimate strength and plastic behavior of reinforced concrete,

Transactions of AIJ, 1951.


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No

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

FRP Reinforcement

593

TABLE MECHANICAL PROPERTIES OF CFRP REINFORCEMENT

Nominal Ultimate

Elastic modulus

Type

Strength

(kN)

(GPa)

Carbon

FRP

96 (

10.5}

140

Strand cable

57 ( t 7.5)

Carbon

FRP

31 (


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594

Kakizawa, Ohno, and Yonezawa

Ctacking

Name of

load (kN)

No

~

pecinens

Calc.

I &SO 5.1

4.2

2CA::

4.1

4.2

3

CPC69II

12.4

13.4

4 IXS8B

12.3

11.9

s

a>c3llll

9.5

9.2

6 CPQi8U 10.4 11.9

~ Y Y

7.4

7.4

s ~ m

7.5 7.4

~ m

7.1

7.4

I O ~ y y

9.5

9.2

u p a m m

9.5

9.2

12 Cl fCB.B.YY

8.4

9.2

13 Cl fCB.B.m

8.0

9.2

14 CPfCllllli.YY

8.5

9.2

S

CPfCllllll.m

9.1

9.2

16,..,...,.........,

11.5

9.2

TABLE 4 TEST RESULTS

MaxinJm

Defteldion

nelgy

absorption

lctkm)

load (kN)

allhe

llliiiiSII8d

miXilun

oad

before altar

Calc.

II1IICirun

I Xi Tur

taal

value

mm)

load load

14.6 12.3 49.91 65.7>

65.7>

35.3 26.0

-

79.3 0.0 79.3

28.9

.25.7

21.99

50.3

6.9

5 7 ~

29.5 25.8 29.36 64.4 0.0 64.4

20.3 15.7 23.76 36.4 0.0

36.4

15.3 18.0 21.36 29.0 30.2 59.2

31.0

26.5

35.56

71.7 29.5

101.2

36.1 27.1 40.27 89.9 3.9 93.9

31.6

25.5

38.01 74.9 12.1 87.1

31.4 24.5 32.70 70.6 39.8

110.4

36.5

25.5

36.24 87.3

0.0

87.3

24.3

25.5

26.29 44.6

109.5

154.1

31.4

25.5

38.41 80.0 10.9

90.9

25.8 22.5 25.32 45.0 48.8 93.8

30.8 24.5 25.33 84.5 0.0

84.5

33.5 25.5 36.23 90 .4 4.5 94.9

I I 1 I I

I -

t 50 +

700

1000

Failure mode

Experimental

Calcurated

observation prediction

Yeild ol steel bals

Yeild of steel bals

Failn

d

IBnSie

ban;

Fabe

d

Tensile

IJus

B8iiiiiced

iiiiiure

Failure d PS

cabl8-

PS cable failure

Balanced

laiure

a11ar lalure

Failure

d

PS cable

Failure d

PS cable

~ l a i u r e ~ l a i u r e

~ l i u r e

~ f l i b

Failure ollensile bals

CorTpession

falJre

& CQIIllf8SSion

lalure

Failure ol PS cable

~ l a i u r e

ancllonsie bals

Failure ol

PS cable

FaiUe

of

PS

cable

and lansile bals

Failure

ol PS

cable

Balanced laiure

ancllansile bals

Falun of PS cable

~ l a i u r e

Balanced laiure

~ l a i u r e

Faiure of PS cable Failure

d

PS cable

Failure

of

PS cable FaiUa

of

PS cable

Faiure of PS cable

~ l a i u r e

100 .j

PC strand

CFRP

cable)

Fig. 1 Details of test specimens


11/14

FRP Reinforcement

9

No.1 RC-SD

No.9 CPRC24UB-YR

I

Yl d{lliafj \ \

zs zs

I mrCwJh\ Y I

s zs

No.2 CRC

No.10 CPRC38BB-YY

I )

r

l\1 \\ r I

zs zs

No.11 CPRC38BB-YR

I r ~ \ I

zs zs

No.12 CPRC38UB-YY

r r)

t

zs zs

No.6 CPC58U

I

r

rhj 1

\\1 \J/

I

zs zs

No.13 CPRC38UB-YR

I c J d i ~ \ \ I

zs zs

zs zs

No.14 CPRC38BU-YY

No.7 CPRC24BB-YY

1cc

rflliYr \

zs

l

No.8 CPRC24BB-YR

Fig 2 Crack patterns of loaded beams


12/14

596 Kakizawa, Ohno, and Yonezawa

0

-

5

E

10

c

1 5

J)

E

J)

20

)

~

Q_

25

0

30

- ~ - - - - - - - - . .

CFRC

+ + =

~ ~ ~ ~

3 5 ...........................T ' ' '(' '

-

*- CPRC38BB-YY

40 L _ ~ ~ ~ J _ ~ ~ ~ L ~ ~ ~ ~ ~ ~ ~ ~

0

42.5 85 127.5 170

Location em)

Fig. 3-Distribution of displacement

at

the load of

5

kN


13/14

40r----------------------

z

~ 3 0

20

CRC

~

-

RC-50

10 20

3 4 5

60

eflection

mm)

a )

40r----------------------

~ 3 0

20

CPRC24BB YR

.../\

/

//_.

/.

r

~ / ~

CPRC24UB YR

CPRC24BB YY

O k ~ ~ ~ ~ ~ ~ _ J

z

~ 3 0

20

0 10 20

3 4

5 60

eflection

mm)

c )

CPRC38UB YY

C P R C 3 8 B U - Y Y ~ ~

O L ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

0 10 2 3 40 5 60 7 8 90

eflection

mm)

e )

RP Reinforcement

597

40r----------------------

~ 3 0

-o

c

2

3 10

CPC69B CPC58B

CPC38B

/ /

... : ~

I

..

_ ; ; - - - - - - ~ - - - . . - -

CPC58U

O L ~ ~ ~ ~ ~ ~ ~

z

~ 3

0 10

2 3

40

5

60

eflection mm)

b )

O L ~ ~ ~ ~ ~ ~ ~

0 10 20

3

40

5

60

eflection

mm)

d )

40 r - - - - - - - - - - - - - - - - - - - - - - ,

z

~ 3

20

CPRC38UB YR

o ~ ~ ~ ~ ~ ~ ~ ~ ~

0 10 20 3 40 5 60

eflection mm)

f )

Fig. 4--Load-deflection curves


14/14

98 Kakizawa Ohno and Yonezawa

0 20 40

60

80 100 120 140 160

Energy Absorption kN-cm)

Fig.

5 Comparison

of energy absorption

Sp138-35 Energy Absorption FRP Reinforced-beams

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